Nowadays, display devices that form a visible image in the air to display required information have been developed. For example, Patent Literatures (PTL) 1 and 2 cited below disclose a visible image forming apparatus that intermittently emits an invisible laser beam, and generates plasma by collecting the laser beam with lenses and mirrors, to thereby form the visible image representing characters and pictures in the air, on the basis of the visible light from the plasma thus generated.
In addition, PTL 3 cited below proposes an in-vehicle aerial imaging apparatus that forms a visible image in a space inside a vehicle to thereby display required information. The in-vehicle aerial imaging apparatus is configured to display the information required by the passenger of the vehicle at a position inside the vehicle interior that is easy for the passenger to visually recognize, instead of on a display panel attached inside the vehicle interior.
Further, an optical imaging apparatus according to PTL 4 cited below is configured to receive and reflect light from an object or a display device, and converge the reflected light at a position symmetrical to the object or the display device with respect to the optical image forming device, to thereby form an object image in the air.